The present invention relates to an ignition apparatus for an internal combustion engine, and more particularly, it relates to an ignition apparatus which is capable of preventing malfunctions due to noise induced by a high voltage generated upon discharge of a spark plug.
FIG. 3 shows a typical example of a known ignition apparatus for an internal combustion engine. In this figure, the apparatus illustrated includes a controller 1 in the form of a control unit for controlling the fuel injection timing and the ignition timing of an internal combustion engine in synchronism with the rotation thereof, a power transistor 2, an ignition coil 3, a reverse-current checking diode 4, and a spark plug 5. The ignition coil 3 has a primary winding connected to ground through the collector-emitter connection of the power transistor 2, and a secondary winding connected to one electrode of the spark plug 5 through the reverse-current checking diode 49 The spark plug electrode is also connected to a negative electrode of a DC power source 8 through a ion current sensing diode 6 and a resistor 7. A serial connection of a capacitor 9 and a resistor 10 is connected in parallel with a serial connection of the resistor 7 and the DC power source 8. A comparator 11 has a pair of first and second input terminals, the first input terminal being connected to a junction between the capacitor 9 and the resistor 10, and the second input terminal being connected to a reference voltage source. When a voltage imposed on the first input terminal exceeds the reference voltage at the second input terminal, the comparator 11 generates an output signal which is input as a reset signal to a pair of first and second counters 12, 13 which together constitute a binary counter. The first counter 12 is alternately actuated and deactuated or turned into a high and a low level by a clock pulse supplied thereto from a signal generator 19 through a comparator 20, which will be described in detail later, and it is reset by a reset signal from the comparator 11, so that it generates an output signal, as shown at (F) in FIG. 4. The second counter 13 generates a high output when a clock signal A is input to the first counter 12 during the time the first counter 12 is at a high level, and it is reset by a reset signal E from comparator 11.
The control unit 1 supplies a fuel injection signal to a fuel injector 14 which injects, based thereon, an appropriate amount of fuel into an intake pipe IP of the engine. The engine includes a cylinder 15 in which a piston 16 is received for reciprocating movement. The piston 16 is connected with a crankshaft 18 through a piston rod 17.
The signal generator 19 generates a control signal in synchronism with the rotation of the crankshaft 18. The control signal contains a series of pulses occurring at predetermined intervals. The control signal from the signal generator 19 is fed to the control unit 1 as well as the first counter 12 through the comparator 20 as a clock signal.
The operation of the above-mentioned known ignition apparatus will now be described in detail with reference to FIG. 4 which is a timing chart showing the waveforms of signals at various portions of the ignition apparatus.
Under the normal operating condition of the engine in which normal combustion takes place in the cylinder 15 without misfiring, in synchronization with an output or clock signal A from the signal generator 19, which is shown at (A) in FIG. 4, the control unit 1 generates a fuel injection control signal to the injector 14 and at the same time, it turns the power transistor 2 off so that a positive voltage is developed across the primary winding of the ignition coil 3, as shown at (B) in FIG. 4, and a negative voltage is developed across the secondary winding of the ignition coil 3, as shown in at (C) in FIG. 4, thereby causing the spark plug 5 to generate a spark. Upon sparking of the spark plug 5, an air/fuel mixture in the cylinder 15 is fired. As a result, between the electrodes of the spark plug 5 there is generated an ion current I which is supplied to the first input terminal of the comparator 11 through the diode 6 and the capacitor 9. The waveform of the ion current I thus supplied to the comparator 11 contains noise N, as illustrated at (D) in FIG. 4, which results from a high voltage generated upon sparking of the spark plug 5. When the comparator 11 receives the ion current I containing noise N at the first input terminal thereof, it generates an output signal in the form of a reset signal E, as shown at (E) in FIG. 4. In other words, within one period of the clock signal A from the signal generator 19, there is generated two types of reset signals, one type being due to noise and the other due to the ion current. As a consequence, the first counter 12, which is alternatively actuated and deactuated by a clock signal pulse and is reset by a reset signal pulse, is always reset by a reset signal due to noise, so that it generates an output signal F which rises at the rising edge of a clock pulse A and falls at the rising edge of a noise-induced reset pulse, as shown at (F) in FIG. 4. Accordingly, the second counter 13 generates no output or a low level output at all times, as shown at (G) in FIG. 4.
In this manner, the first and second counters 12, 13 of the known ignition apparatus operate irrespective of the presence and absence of an ion current, so when misfiring takes place at a time between time t2 corresponding to the rising edge of a square clock pulse and time t3 corresponding to the rising edge of the following clock pulse, it is impossible to detect this misfiring.